1. Field of the Invention
The present invention relates to a liquid crystal display device, and a light source driving circuit and method to be used in the liquid crystal display device, and more particularly to the liquid crystal display device having a function, such as a multi-sync function, of operating in a case when frequencies of a vertical sync signal and horizontal sync signal contained in a video input signal are changed whenever necessary, and the light source driving circuit and the light source driving method to be respectively used in the liquid crystal display device.
The present application claims priority of Japanese Patent Application No. 2004-136331 filed on Apr. 30, 2004, which is hereby incorporated by reference.
2. Description of the Related Art
In a liquid crystal display device, as a light source (for example, a backlight) to illuminate a liquid crystal panel, a discharge lamp such as a cold cathode tube is used in many cases. The discharge lamp is lit when a high-voltage alternating current is fed. The high-voltage alternating current is produced by a resonant circuit made up of an inductor of a transformer in an inverter and a capacitor, and efficiency of the resonant circuit differs depending on a frequency of the high-voltage alternating current. Higher efficiency is obtained when the resonant circuit operates in the vicinity of a resonant frequency. Recently, the liquid crystal display device is widely used in personal computers, televisions, or a like, as a screen displaying means and has a function, such as a multi-sync function, of operating in a manner to correspond to a vertical sync signal and horizontal sync signal with various frequencies. However, the conventional liquid crystal display device has a problem in that, in the case when a driving frequency of a discharge lamp is fixed at a resonant frequency that enables the resonant circuit to operate in an efficient manner, when frequencies of a vertical sync signal and horizontal sync signal contained in a video input signal are changed, flicker and/or fringes caused by interference with the driving frequency of the discharge lamp are visually recognized on a display screen of the liquid crystal display device. To solve this problem, conventional technologies are proposed.
For example, a backlight driving circuit is disclosed in Japanese Patent Application Laid-open No. 2002-8887 in which an oscillation circuit 1 has, as shown in FIG. 15, an LC resonant circuit made up of an inductance device and a capacitance device, and operates at a resonant frequency of the LC resonant circuit. Then, a driving signal having a resonant frequency of the LC resonant circuit is fed from the oscillation circuit 1 to a backlight 2. Moreover, a horizontal frequency of an input video signal “in” is detected by a microcomputer 3 and an oscillation frequency of the oscillation circuit 1 is calibrated according to the horizontal frequency. That is, if the detected horizontal frequency is at a specified threshold value or less, capacitance or inductance of the above LC resonant circuit is switched so that the oscillation frequency exceeds the threshold value. Also, if the detected horizontal frequency is at a specified threshold value or more, the capacitance or inductance of the LC resonant circuit is switched so that an oscillation frequency becomes the threshold value or less and, as a result, the horizontal frequency is switched, however, flicker and fringes caused by the interference with a driving frequency of the backlight 2 are not readily visually recognized on a display screen of a liquid crystal display device.
Also, a liquid crystal display device provided with a backlight is disclosed in Japanese Patent Application Laid-open No. Hei 05-113766 which includes, as shown in FIG. 16, an F-V (Frequency-Voltage) converter 11, a voltage controlling circuit 12, an oscillation circuit 13, a boosting transformer 14, and a fluorescent lamp (used as the backlight) 15. In the liquid crystal display device, a frequency of a horizontal sync signal “c” contained in a video signal is detected by the F-V converter 11 and an oscillation frequency of the oscillation circuit 13 is made by the voltage controlling circuit 12 to be variable according to the detected frequency and a lighting frequency of the fluorescent lamp 15 through the boosting transformer 14 is changed. As a result, a flicker caused by interference between a driving frequency for the liquid crystal display device and a lighting frequency of the fluorescent lamp (backlight) 15 disappears from a display screen. Additionally, even if the lighting frequency is changed, a power source voltage is made variable so that luminance of the fluorescent lamp 15 becomes constant.
However, the above conventional technologies have the following problems. That is, in the backlight driving circuit disclosed in the Japanese Patent Application Laid-open No. 2002-8887, when a resonant frequency on the transformer's primary side of the LC resonant circuit making up the oscillation circuit 1 is changed, the changed resonant frequency does not coincide with a frequency on the transformer's secondary side, which causes a problem in that efficiency of the LC resonant circuit is degraded.
Moreover, in the liquid crystal display device with the backlight disclosed in the Japanese Patent Application Laid-open No. Hei 05-113766, a lighting frequency of the fluorescent lamp 15 as backlight is changed based on the horizontal sync signal “c”, whereas the flicker caused by interference between the driving frequency of the liquid crystal display device and the lighting frequency of the fluorescent lamp 15 occurs not only due to interference between the lighting frequency of the fluorescent lamp 15 and the horizontal sync signal “c” used in the liquid crystal display device, but due to interference between the lighting frequency of the fluorescent lamp 15 and the vertical sync signal used in the liquid crystal display device. Therefore, even if only the horizontal sync signal “c” is detected, ripples are visually recognized in some cases. Also, there is a problem in that the efficiency of the oscillation circuit 13 is degraded due to the change in the lighting frequency of the fluorescent lamp 15.